Summary of work: Several cellular markers of oxidative stress are elevated in cells and tissue samples from Alzheimer disease (AD) patients as compared to normal age-matched controls. These markers include oxidative damage to lipids, proteins and DNA. This correlation suggests that AD pathology may be associated with or predisposed by a defect in the DNA repair processing of oxidative base lesion leading to accumulation of DNA damage. We are testing this hypothesis utilizing several biological models, including AD patients? post-mortem tissue. Using DNA substrates containing both pyrimidine and purine lesions we have investigated the repair of oxidative base lesions in whole cell extracts from cultured AD lymphoblasts. Our data indicate that certain oxidative DNA lesions are repaired as efficiently in AD lymphoblast as in controls, which indicated that the alterations in oxidative damage processing may be highly cell type-specific. In normal cells, oxidative DNA damage is mainly repaired by the base excision repair (BER) pathway. We thus measured BER capacity in tissue extracts obtained from well established animal models for AD. We have used three mouse model systems for AD, transgenic mice expressing mutant amyloid precursor protein 1 (APP1) gene; a double transgenic mouse expressing mutant APP1 plus mutant presenilin 1; and a triple transgenic mouse expressing the two previous genes plus a mutated form of tau. All these gene products are involved in the formation of plaques and tangles in the AD brain and these mice develop several AD-like symptoms in an age-associated fashion. Thus, we compare DNA repair activities in young and old mice, i.e. before and after the onset of the disease. Moreover, because some regions of the brain are pathologically affected (for example corpus callosum and hippocampus atrophy) while other regions seem to remain unaffected, we are measuring repair capacity in extracts of 5 different brain regions in normal and AD-model mice. We also follow age-associated changes in DNA repair capacity in these regions in wild type mice. Our results show that BER activities in mitochondria varied greatly among striatum, frontal cortex, cerebellum, hippocampus and brain steam, with brain steam having highest and striatum the lowest DNA glycosylase activities. We observed a general decrease in BER efficiency in brain with age; however the age-associated changes also differ among the regions. In contrast, we observed decreased activity for some BER enzymes, but not all, and this was restricted to two regions of the brains of older AD mice when compared with young, pre-symptomatic mice. The regions with altered BER activity did not correlate with the pathologically affected ones and we are now investigating whether this is due to cell type-specific sensitivity to environmental factors. Nonetheless, mice do not reflect all the pathological hallmarks of AD in humans, thus we are measuring BER activities in post-mortem tissue samples from AD patients and age-matched cognitive normal controls. Alternatively, we are directly testing the hypothesis that accumulation of oxidative DNA damage (8-oxodG) plays a role in neurodegenerative processes. For that, mice deficient in the oxoguanine DNA glycosylase (OGG1) are subjected to brain ischemia-reperfusion models. These animals completely lack 8-oxoG removal in mitochondria and show decrease activity in the nuclei. We find that OGG1-/- mice develop a larger infarct area after ischemia-reperfusion and this correlates with a higher degree of motor dysfunction. This direct evidence that 8-oxoG accumulation sensitizes neurons to oxidative stress-induced cell death, together with the correlative changes in BER capacity in AD samples indicate that changes in DNA repair and DNA damage response may play a direct role in the development of neurodegenerative diseases.